Abrasion-resistant steel plate and method of producing abrasion-resistant steel plate

ABSTRACT

Provided is an abrasion-resistant steel plate excellent in both abrasion resistance and wide bending workability. An abrasion-resistant steel plate comprises a specific chemical composition, wherein a volume fraction of martensite at a depth of 1 mm from a surface of the abrasion-resistant steel plate is 90% or more, hardness at a depth of 1 mm from the surface is 420 HBW 10/3000 to 560 HBW 10/3000 in Brinell hardness, and a transverse direction hardness difference is 30Hv10 or less in Vickers hardness, the transverse direction hardness difference being defined as a difference in the hardness at a depth of 1 mm from the surface between two points adjacent at intervals of 10 mm in a transverse direction of the abrasion-resistant steel plate.

TECHNICAL FIELD

The present disclosure relates to an abrasion-resistant steel plate, andparticularly relates to an abrasion-resistant steel plate havingexcellent wide bending workability and suitable for members ofindustrial machinery and transportation equipment used in the fields ofconstruction, civil engineering, mining, etc. The present disclosurealso relates to a method of producing the abrasion-resistant steelplate. The term “wide bending workability” herein denotes bendingworkability for a steel plate width of 200 mm or more, which is an issuein actual use.

BACKGROUND

It is known that the abrasion resistance of a steel material is improvedby increasing its hardness. Hence, steel materials subjected to heattreatment such as quenching to have higher hardness are used in membersexposed to abrasion by earth and sand, rocks, and the like.

For example, JP S63-169359 A (PTL 1) describes a method of producing anabrasion-resistant steel plate by hot rolling a steel material having aspecific chemical composition to obtain a steel plate and then quenchingthe steel plate. With the method described in PTL 1, by controlling thecontents of C, alloying elements, and N, an abrasion-resistant steelplate that has a hardness of 340 HB or more and high toughness asquenched and has improved weld low-temperature cracking resistance isobtained.

JP S64-031928 A (PTL 2) describes a method of producing anabrasion-resistant steel plate by hot rolling a steel having a specificchemical composition at a temperature of 900° C. to Ar3 transformationpoint with a reduction ratio of 15% or more and then direct quenchingthe obtained steel plate from a temperature of Ar3 transformation pointor more. With the method described in PTL 2, by controlling the chemicalcomposition and the quenching conditions, an abrasion-resistant steelplate having high hardness can be obtained easily.

The respective techniques described in PTL 1 and PTL 2 improve theabrasion resistance by increasing the hardness. Meanwhile, there is alsoa growing demand for abrasion-resistant steels excellent not only inabrasion resistance but also in bending workability, for application tomembers of various shapes and reduction of welded portions.

In response to such demand, for example, JP H07-090477 A (PTL 3)proposes an abrasion-resistant steel containing, in wt%, C: 0.05% to0.20%, Mn: 0.50% to 2.5%, and Al: 0.02% to 2.00% and having an areafraction of martensite of 5% or more and 50% or less. According to PTL3, a hot-rolled steel is heated to a temperature in a ferrite-austenitedual phase region between Ac1 point and Ac3 point and then rapidlycooled to control the area fraction of martensite, as a result of whichan abrasion-resistant steel having excellent workability and weldabilityis obtained.

JP 2006-104489 A (PTL 4) proposes a method of producing anabrasion-resistant steel plate by hot rolling a steel having a specificchemical composition, then immediately cooling the obtained steel plateto Ms point ±25° C., stopping the cooling and recuperating the steelplate to Ms point +50° C. or more, and then cooling the steel plate toroom temperature. According to PTL 4, the minimum hardness in a regionfrom the surface to 5 mm in depth of the steel plate obtained by thisproduction method is at least 40 HV less than the maximum hardness in amore internal region of the steel plate, and thus the bendingworkability is improved.

JP 2008-169443 A (PTL 5) proposes a method of producing anabrasion-resistant steel plate by hot rolling a steel having a specificchemical composition with DI* (quench hardenability index) of 60 or moreand then cooling the obtained steel plate to a temperature range of 400°C. or less at an average cooling rate of 0.5° C./s to 2° C./s. Accordingto PTL 5, 400 particles/mm² or more of Ti-based carbide with an averageparticle size of 0.5 μm to 50 μm precipitate in the abrasion-resistantsteel plate obtained by this production method, and thusabrasion-resistant steel having excellent abrasion resistance andbending workability is obtained without heat treatment.

CITATION LIST Patent Literature

PTL 1: JP S63-169359 A

PTL 2: JP S64-031928 A

PTL 3: JP H07-090477 A

PTL 4: JP 2006-104489 A

PTL 5: JP 2008-169443 A

SUMMARY Technical Problem

As described in PTL 3 to PTL 5, the conventional methods of improvingthe bending workability of an abrasion-resistant steel plate are basedon the concept that, while ensuring the bending workability by limitingthe hardness of the matrix of the steel plate, the abrasion resistanceis improved by microstructure control or carbide precipitation. Withsuch methods, it is difficult to sufficiently improve the hardness ofthe matrix, making it impossible to achieve both the abrasion resistanceand the bending workability.

Given that the demand level of abrasion resistance is increasing year byyear, there is a need for a technique capable of achieving both theabrasion resistance and the bending workability, which are mutuallycontradictory properties, at high level.

When working an abrasion-resistant steel plate to produce a finishedproduct such as a member for civil engineering and constructionequipment, bending work is typically performed under a condition thatthe plate width of the abrasion-resistant steel plate is 200 mm or more.Since bending cracks are usually more likely to occur when the platewidth is wider, a steel plate with a plate width of 200 mm or more needsto be used to evaluate the bending workability of the steel plate inactual use. The bending workability for a plate width of 200 mm or moreis, however, not taken into consideration in the foregoing conventionaltechniques.

It could therefore be helpful to provide an abrasion-resistant steelplate excellent in both abrasion resistance and bending workabilitywhich are mutually contradictory properties. With regard to the bendingworkability, in particular, it could be helpful to provide anabrasion-resistant steel plate having excellent bending workabilityunder a severe condition that the steel plate width is 200 mm or more(hereafter referred to as “wide bending workability”).

Solution to Problem

We studied each factor that influences the wide bending workability ofan abrasion-resistant steel plate, and consequently discovered thefollowing (1) to (4).

-   -   (1) The bending workability of the abrasion-resistant steel        plate is significantly influenced by the hardness and ductility        of the surface layer of the abrasion-resistant steel plate.    -   (2) In particular, if the abrasion-resistant steel plate has a        locally hardened zone or softened zone, strain concentrates        around the softened zone or hardened zone and the ductility        decreases, so that the wide bending workability decreases.    -   (3) By reducing the difference in hardness in the        abrasion-resistant steel plate, the wide bending workability can        be improved without decreasing the hardness of the matrix which        significantly affects the abrasion resistance.    -   (4) In the production of the abrasion-resistant steel plate, by        performing quenching from the austenite temperature range and        reducing the difference in cooling rate in the transverse        direction of the steel plate during the quenching, the        difference in hardness in the abrasion-resistant steel plate can        be reduced.

The present disclosure is based on these discoveries and furtherstudies. We thus provide the following.

-   -   1. An abrasion-resistant steel plate comprising a chemical        composition containing (consisting of), in mass%, C: 0.15% to        0.30%, Si: 0.05% to 1.00%, Mn: 0.50% to 2.00%, P: 0.020% or        less, S: 0.010% or less, Al: 0.01% to 0.06%, Cr: 0.10% to 1.00%,        and N: 0.0100% or less, with a balance consisting of Fe and        inevitable impurities, wherein a volume fraction of martensite        at a depth of 1 mm from a surface of the abrasion-resistant        steel plate is 90% or more, hardness at a depth of 1 mm from the        surface is 420 HBW 10/3000 to 560 HBW 10/3000 in Brinell        hardness, and a transverse direction hardness difference is        30Hv10 or less in Vickers hardness, the transverse direction        hardness difference being defined as a difference in the        hardness at a depth of 1 mm from the surface between two points        adjacent at intervals of 10 mm in a transverse direction of the        abrasion-resistant steel plate.    -   2. The abrasion-resistant steel plate according to 1., wherein        the chemical composition further contains, in mass%, one or more        selected from the group consisting of Nb: 0.005% to 0.020%, Ti:        0.005% to 0.020%, and B: 0.0003% to 0.0030%.    -   3. The abrasion-resistant steel plate according to 1. or 2.,        wherein the chemical composition further contains, in mass%, one        or more selected from the group consisting of Cu: 0.01% to 0.5%,        Ni: 0.01% to 3.0%, Mo: 0.1% to 1.0%, V: 0.01% to 0.10%, W: 0.01%        to 0.5%, and Co: 0.01% to 0.5%.    -   4. The abrasion-resistant steel plate according to any one of 1.        to 3., wherein the chemical composition further contains, in        mass%, one or more selected from the group consisting of Ca:        0.0005% to 0.0050%, Mg: 0.0005% to 0.0100%, and REM: 0.0005% to        0.0200%.    -   5. A method of producing an abrasion-resistant steel plate, the        method comprising: heating a steel material to a heating        temperature that is an Ac3 transformation point or more and        1300° C. or less, the steel material having a chemical        composition containing, in mass%, C: 0.15% to 0.30%, Si: 0.05%        to 1.00%, Mn: 0.50% to 2.00%, P: 0.020% or less, S: 0.010% or        less, Al: 0.01% to 0.06%, Cr: 0.10% to 1.00%, and N: 0.0100% or        less, with a balance consisting of Fe and inevitable impurities;        hot rolling the heated steel material to obtain a hot-rolled        steel plate; and subjecting the hot-rolled steel plate to        quenching, wherein the quenching is (a) direct quenching of        cooling the hot-rolled steel plate from a cooling start        temperature that is an Ar3 transformation point or more to a        cooling stop temperature that is a Mf point or less or (b)        reheating quenching of cooling the hot-rolled steel plate,        reheating the cooled hot-rolled steel plate to a reheating        temperature that is the Ac3 transformation point or more and        950° C. or less, and cooling the reheated hot-rolled steel plate        from the reheating temperature to a cooling stop temperature        that is the Mf point or less, and in a cooling process in the        quenching, a difference in average cooling rate between a center        position and a ¼ position of the hot-rolled steel plate in the        transverse direction and a difference in average cooling rate        between the center position and a ¾ position of the hot-rolled        steel plate in the transverse direction are each 5° C./s or        less.    -   6. The method of producing an abrasion-resistant steel plate        according to 5., wherein the cooling stop temperature in the        quenching is less than (Mf point −100° C.), and the method        comprises, after the quenching, tempering the quenched        hot-rolled steel plate at a tempering temperature that is (Mf        point −80° C.) or more and (Mf point +50° C.) or less.    -   7. The method of producing an abrasion-resistant steel plate        according to 6., wherein in the tempering, the quenched        hot-rolled steel plate is held at the tempering temperature for        60 s or more.    -   8. The method of producing an abrasion-resistant steel plate        according to 6. or 7., wherein an average heating rate in the        tempering is 2° C./s or more.    -   9. The method of producing an abrasion-resistant steel plate        according to 5., wherein the cooling stop temperature in the        quenching is Mf point or less and (Mf point −100° C.) or more,        and the method comprises, after the quenching, air cooling the        quenched hot-rolled steel plate.    -   10. The method of producing an abrasion-resistant steel plate        according to any one of 5. to 9., wherein the chemical        composition further contains, in mass%, one or more selected        from the group consisting of Nb: 0.005% to 0.020%, Ti: 0.005% to        0.020%, and B: 0.0003% to 0.0030%.    -   11. The method of producing an abrasion-resistant steel plate        according to any one of 5. to 10., wherein the chemical        composition further contains, in mass%, one or more selected        from the group consisting of Cu: 0.01% to 0.5%, Ni: 0.01% to        3.0%, Mo: 0.1% to 1.0%, V: 0.01% to 0.10%, W: 0.01% to 0.5%, and        Co: 0.01% to 0.5%.    -   12. The method of producing an abrasion-resistant steel plate        according to any one of 5. to 11., wherein the chemical        composition further contains, in mass%, one or more selected        from the group consisting of Ca: 0.0005% to 0.0050%, Mg: 0.0005%        to 0.0100%, and REM: 0.0005% to 0.0200%.

Advantageous Effect

It is thus possible to produce an abrasion-resistant steel plateexcellent in both abrasion resistance and wide bending workability.Since excellent wide bending workability can be achieved without adecrease in hardness which affects the abrasion resistance, high demandlevel of abrasion resistance in recent years can be satisfied. Theabrasion-resistant steel plate according to the present disclosure istherefore suitable as material for members of industrial machinery andtransportation equipment used in the fields of construction, civilengineering, mining, etc.

DETAILED DESCRIPTION

The following will describe embodiments of the present disclosure indetail. The following description shows examples of preferredembodiments of the present disclosure and does not limit the scope ofthe present disclosure.

Chemical Composition

In the present disclosure, it is important that an abrasion-resistantsteel plate and a steel material used in the production of theabrasion-resistant steel plate have the above-described chemicalcomposition. First, the reasons for limiting the chemical composition ofthe steel as described above in the present disclosure will be describedbelow. Herein, “%” with regard to the chemical composition is mass%unless otherwise stated.

C: 0.15% to 0.30%

C is an element that increases the hardness of the matrix and improvesthe abrasion resistance. To achieve this effect, the C content is 0.15%or more. The C content is preferably 0.20% or more. If the C content ismore than 0.30%, the hardness of the matrix increases excessively, andthe wide bending workability greatly decreases. The C content istherefore 0.30% or less. The C content is preferably 0.28% or less.

Si: 0.05% to 1.00%

Si is an element that acts as a deoxidizer. Si also has the effect ofincreasing the hardness of the matrix by solid solution strengthening inthe steel. If the Si content is less than 0.05%, the deoxidizing effectis insufficient and the amount of inclusions increases, and as a resultthe ductility decreases. The Si content is therefore 0.05% or more. TheSi content is preferably 0.10% or more, and more preferably 0.20% ormore. If the Si content is more than 1.00%, the amount of inclusionsincreases and the ductility decreases, and as a result the wide bendingworkability decreases. The Si content is therefore 1.00% or less. The Sicontent is preferably 0.80% or less, and more preferably 0.60% or less.

Mn: 0.50% to 2.00%

Mn is an element that increases the hardness of the matrix and improvesthe abrasion resistance. If the Mn content is less than 0.50%, thequench hardenability is insufficient, and uniform hardness cannot beachieved. The Mn content is therefore 0.50% or more. The Mn content ispreferably 0.60% or more, and more preferably 0.70% or more. If the Mncontent is more than 2.00%, the hardness increases excessively, so thatthe wide bending workability decreases. The Mn content is therefore2.00% or less. The Mn content is preferably 1.80% or less, and morepreferably 1.60% or less.

P: 0.020% or Less

P is an element contained as an inevitable impurity, and has an adverseeffect such as segregating to grain boundaries and acting as a fractureorigin. Accordingly, it is desirable to reduce the P content as much aspossible, but 0.020% or less is acceptable. Although no lower limit isplaced on the P content, reducing the P content to less than 0.001% isdifficult in industrial scale production. Hence, the P content ispreferably 0.001% or more from the viewpoint of productivity.

S: 0.010% or Less

S is an element contained as an inevitable impurity, and has an adverseeffect such as existing in the steel as a sulfide-based inclusion suchas MnS and acting as a fracture origin. Accordingly, it is desirable toreduce the S content as much as possible, but 0.010% or less isacceptable. Although no lower limit is placed on the S content, reducingthe S content to less than 0.0001% is difficult in industrial scaleproduction. Hence, the S content is preferably 0.0001% or more from theviewpoint of productivity.

Al: 0.01% to 0.06%

Al is an element that acts as a deoxidizer and also has the effect offorming nitride to refine crystal grains and improve the ductility. Toachieve these effects, the Al content is 0.01% or more. If the Alcontent is more than 0.06%, nitride forms excessively and surfacedefects increase. If the Al content is more than 0.06%, oxide-basedinclusions increase and the ductility decreases, as a result of whichthe wide bending workability decreases. The Al content is therefore0.06% or less. The Al content is preferably 0.05% or less, and morepreferably 0.04% or less.

Cr: 0.10% to 1.00%

Cr is an element that has the effect of increasing the hardness of thematrix and improving the abrasion resistance. If the Cr content is lessthan 0.10%, the quench hardenability improving effect by adding Crcannot be achieved, and uniform hardness cannot be obtained. The Crcontent is therefore 0.10% or more. The Cr content is preferably 0.20%or more, and more preferably 0.25% or more. If the Cr content is morethan 1.00%, the ductility decreases due to precipitate formation, andthe wide bending workability decreases. The Cr content is therefore1.00% or less. The Cr content is preferably 0.85% or less, and morepreferably 0.80% or less.

N: 0.0100% or less

N is an element contained as an inevitable impurity, and forms nitrideand the like and thus contributes to the refinement of crystal grains.If the precipitate formation is excessive, however, the ductilitydecreases and the wide bending workability decreases. The N content istherefore 0.0100% or less. The N content is preferably 0.0060% or less,and more preferably 0.0040% or less. Although no lower limit is placedon the N content, reducing the N content to less than 0.0010% isdifficult in industrial scale production. Hence, the N content ispreferably 0.0010% or more from the viewpoint of productivity.

The abrasion-resistant steel plate and the steel material according toone embodiment of the present disclosure have a chemical compositioncontaining the above-described components with the balance consisting ofFe and inevitable impurities.

In another embodiment of the present disclosure, the chemicalcomposition may optionally further contain one or more selected from thegroup consisting of Nb: 0.005% to 0.020%, Ti: 0.005% to 0.020%, and B:0.0003% to 0.0030%.

Nb: 0.005% to 0.020%

Nb is an element that increases the hardness of the matrix andcontributes to further improvement in abrasion resistance. Nb also formscarbonitride and refines prior austenite grains. In the case of addingNb, to achieves these effects, the Nb content is 0.005% or more, andpreferably 0.007% or more. If the Nb content is more than 0.020%, NbCprecipitates in large amount and the ductility decreases, as a result ofwhich the wide bending workability decreases. Accordingly, in the caseof adding Nb, the Nb content is 0.020% or less. The Nb content ispreferably 0.018% or less.

Ti: 0.005% to 0.020%

Ti is an element that forms nitride in the steel and refines prioraustenite grains to thus improve the ductility. In the case where Ti andB coexist, as a result of Ti fixing N, the precipitation of BN issuppressed, with it being possible to enhance the quench hardenabilityimprovement effect by B. In the case of adding Ti, to achieve theseeffects, the Ti content is 0.005% or more. The Ti content is preferably0.007% or more. If the Ti content is more than 0.020%, hard TiCprecipitates in large amount, causing a decrease in wide bendingworkability. Accordingly, in the case of adding Ti, the Ti content is0.020% or less. The Ti content is preferably 0.015% or less.

B: 0.0003% to 0.0030%

B is an element that greatly improves the quench hardenability even whenadded in small amount. By adding B, the formation of martensite can bepromoted and the abrasion resistance can be improved more effectively.In the case of adding B, to achieve this effect, the B content is0.0003% or more. The B content is preferably 0.0005% or more, and morepreferably 0.0008% or more. If the B content is more than 0.0030%, anadverse effect such as forming a large amount of precipitates, such asboride, that act as a fracture origin occurs. Accordingly, in the caseof adding B, the B content is 0.0030% or less. The B content ispreferably 0.0015% or less.

In another embodiment of the present disclosure, the chemicalcomposition may optionally further contain one or more selected from thegroup consisting of Cu: 0.01% to 0.5%, Ni: 0.01% to 3.0%, Mo: 0.1% to1.0%, V: 0.01% to 0.10%, W: 0.01% to 0.5%, and Co: 0.01% to 0.5%.

Cu: 0.01% to 0.5%

Cu is an element that improves the quench hardenability, and may beoptionally added in order to further improve the hardness. In the caseof adding Cu, to achieve this effect, the Cu content is 0.01% or more.If the Cu content is more than 0.5%, surface defects tend to occur,causing a decrease in productivity. Moreover, the alloy cost increases.Accordingly, in the case of adding Cu, the Cu content is 0.5% or less.

Ni: 0.01% to 3.0%

Ni is an element that improves the quench hardenability, and may beoptionally added in order to further improve the hardness. In the caseof adding Ni, to achieve this effect, the Ni content is 0.01% or more.If the Ni content is more than 3.0%, the alloy cost increases.Accordingly, the Ni content is 3.0% or less.

Mo: 0.1% to 1.0%

Mo is an element that improves the quench hardenability, and may beoptionally added in order to further improve the hardness. In the caseof adding Mo, to achieve this effect, the Mo content is 0.1% or more. Ifthe Mo content is more than 1.0%, the weldability degrades and the alloycost increases. Accordingly, in the case of adding Mo, the Mo content is1.0% or less.

V: 0.01% to 0.10%

V is an element that improves the quench hardenability, and may beoptionally added in order to further improve the hardness. V alsoprecipitates as VN and thus contributes to the reduction of solute N. Inthe case of adding V, to achieve these effects, the V content is 0.01%or more. If the V content is more than 0.10%, hard VC precipitates,causing a decrease in ductility. Accordingly, in the case of adding V,the V content is 0.10% or less, preferably 0.08% or less, and morepreferably 0.05% or less.

W: 0.01% to 0.5%

W is an element that improves the quench hardenability as with Mo, andmay be optionally added. In the case of adding W, to achieve thiseffect, the W content is 0.01% or more. If the W content is more than0.5%, the alloy cost increases. Accordingly, in the case of adding W,the W content is 0.5% or less.

Co: 0.01% to 0.5%

Co is an element that improves the quench hardenability, and may beoptionally added. In the case of adding Co, to achieve this effect, theCo content is 0.01% or more. If the Co content is more than 0.5%, thealloy cost increases. Accordingly, in the case of adding Co, the Cocontent is 0.5% or less.

In another embodiment of the present disclosure, the chemicalcomposition may optionally further contain one or more selected from thegroup consisting of Ca: 0.0005% to 0.0050%, Mg: 0.0005% to 0.0100%, andREM: 0.0005% to 0.0200%.

Ca: 0.0005% to 0.0050%

Ca is an element useful for morphological control of sulfide-basedinclusions, and may be optionally added. To achieve this effect, the Cacontent needs to be 0.0005% or more. Accordingly, in the case of addingCa, the Ca content is 0.0005% or more. If the Ca content is more than0.0050%, the ductility decreases due to an increase in the amount ofinclusions in the steel, as a result of which the wide bendingworkability decreases. Accordingly, in the case of adding Ca, the Cacontent is 0.0050% or less, and preferably 0.0025% or less.

Mg: 0.0005% to 0.0100%

Mg is an element that forms oxide, that is stable at high temperature,to effectively suppress coarsening of prior austenite grains and improvethe ductility. To achieve this effect, the Mg content needs to be0.0005% or more. Accordingly, in the case of adding Mg, the Mg contentis 0.0005% or more. If the Mg content is more than 0.0100%, theductility decreases due to an increase in the amount of inclusions inthe steel, as a result of which the wide bending workability decreases.Accordingly, in the case of adding Mg, the Mg content is 0.0100% orless, and preferably 0.0050% or less.

REM: 0.0005% to 0.0200%

REM (rare earth metal) has the effect of forming oxide and sulfide inthe steel and improving the material properties, as with Ca. To achievethis effect, the REM content needs to be 0.0005% or more. Accordingly,in the case of adding REM, the REM content is 0.0005% or more. If theREM content is more than 0.0200%, the effect is saturated. Accordingly,in the case of adding REM, the REM content is 0.0200% or less, andpreferably 0.0100% or less.

Microstructure Volume Fraction of Martensite: 90% or More

In the present disclosure, the volume fraction of martensite at a depthof 1 mm from the surface of the abrasion-resistant steel plate is 90% ormore. If the volume fraction of martensite is less than 90%, thehardness of the matrix of the abrasion-resistant steel plate decreases,so that the abrasion resistance degrades. The volume fraction ofmartensite is therefore 90% or more. Since a higher volume fraction ofmartensite is better, no upper limit is placed on the volume fraction,and the volume fraction may be 100%. The volume fraction of martensitecan be measured by the method described in the EXAMPLES section.

If the volume fraction of martensite is 90% or more, the desiredabrasion resistance can be achieved regardless of the residualmicrostructure. Hence, the residual microstructure other than martensiteis not limited, and may be any microstructure. For example, the residualmicrostructure may be one or more selected from the group consisting offerrite, pearlite, austenite, and bainite.

Hardness Brinell Hardness: 420 HBW 10/3000 to 560 HBW 10/3000

In addition to having the foregoing chemical composition, theabrasion-resistant steel plate according to the present disclosure has ahardness of 420 HBW 10/3000 to 560 HBW 10/3000 in Brinell hardness at adepth of 1 mm from the surface. The reasons for limiting the surfacehardness will be described below.

The abrasion resistance of the steel plate can be improved by increasingthe hardness of the surface layer of the steel plate. If the hardness ata depth of 1 mm from the surface of the steel plate is less than 420 HBWin Brinell hardness, sufficient abrasion resistance cannot be achieved,leading to a shorter use life. Accordingly, the hardness at a depth of 1mm from the surface of the steel plate is 420 HBW or more and preferably440 HBW or more in Brinell hardness. If the hardness at a depth of 1 mmfrom the surface of the steel plate is more than 560 HBW in Brinellhardness, the wide bending workability degrades. Accordingly, thehardness at a depth of 1 mm from the surface of the steel plate is 560HBW or less in Brinell hardness. The Brinell hardness herein is thevalue (HBW 10/3000) measured at a position of ¼ of the plate width usinga tungsten hard ball of 10 mm in diameter with a load of 3000 kgf.

Transverse Direction Hardness Difference Transverse Direction HardnessDifference: 30Hv10 or Less

If the abrasion-resistant steel plate has a locally hardened zone orsoftened zone, strain concentrates around the softened zone or hardenedzone and the ductility decreases, so that excellent wide bendingworkability cannot be achieved. In view of this, in the presentdisclosure, the transverse direction hardness difference is 30Hv10 orless in Vickers hardness. The transverse direction hardness differenceherein is defined as the difference in hardness at a depth of 1 mm fromthe surface of the abrasion-resistant steel plate between two pointsadjacent at intervals of 10 mm in the plate transverse direction. As aresult of the hardness difference being in this range, favorable bendingproperty can be achieved even in wide bending work. Since a steel plateis typically produced while being moved in the rolling direction, ifuniformity is maintained in the transverse direction (i.e. the directionorthogonal to the rolling direction), uniformity is equally maintainedin the rolling direction.

The transverse direction hardness difference can be evaluated by, at adepth position of 1 mm from the surface of the abrasion-resistant steelplate, performing Vickers hardness measurement at intervals of 10 mm inthe transverse direction and calculating the difference in hardnessbetween adjacent measurement points. The expression “the transversedirection hardness difference is 30Hv10 or less” means that the hardnessdifference between every pair of adjacent points is 30Hv10 or less, thatis, the maximum hardness difference between adjacent two points is30Hv10 or less.

For cutting of an abrasion-resistant steel plate, thermal cutting suchas gas cutting, plasma cutting, or laser cutting is typically used. Inthe thermally cut abrasion-resistant steel plate, the hardness at edgeparts has changed due to the influence of heat during the cutting.Hence, the heat-affected zones at the edge parts of theabrasion-resistant steel plate are excluded from the measurement of thetransverse direction hardness difference. In detail, the Vickershardness measurement is performed at intervals of 10 mm in thetransverse direction except a region of 50 mm on each end of theabrasion-resistant steel plate. The transverse direction hardnessdifference can thus be determined.

If the measurement is performed at intervals greater than 10 mm, ahardness change that causes degradation in bending workability cannot bedetected. If the measurement intervals are shorter, the hardness changedetection accuracy increases, but the number of measurement points isenormous. Moreover, it was demonstrated that excellent performance canbe actually achieved by controlling the hardness difference measured atintervals of 10 mm, as described in the EXAMPLES section below. Forthese reasons, the measurement interval is 10 mm.

Plate Thickness

The plate thickness of the abrasion-resistant steel plate according tothe present disclosure is not limited, and may be any plate thickness.Given that abrasion-resistant steel plates of 4 mm to 60 mm in platethickness are particularly required to have wide bending workability,the plate thickness of the abrasion-resistant steel plate is preferably4 mm to 60 mm.

Production Method

A method of producing an abrasion-resistant steel plate according to oneembodiment of the present disclosure will be described below. Theabrasion-resistant steel plate according to the present disclosure canbe produced by heating a steel material having the foregoing chemicalcomposition, hot rolling the steel material, and then subjecting theobtained steel plate to heat treatment including quenching under thebelow-described conditions.

Steel Material

As the steel material, any form of material may be used. For example,the steel material may be a steel slab.

The method of producing the steel material is not limited. For example,the steel material can be produced by smelting a molten steel having theforegoing chemical composition by a conventional method and casting thesteel. The smelting may be performed by any method such as a converter,an electric furnace, or an induction furnace. The casting is preferablyperformed by continuous casting from the viewpoint of productivity, butmay be performed by ingot casting.

Heating

The steel material is heated to a heating temperature prior to hotrolling. The heating may be performed after cooling the steel materialobtained by casting and the like. Alternatively, the obtained steelmaterial may be directly heated without cooling.

Heating Temperature: Ac3 Transformation Point or More and 1300° C. orLess

If the heating temperature is less than Ac3 transformation point,ferrite phase is contained in the microstructure of the steel plateafter the heating. In such a case, not only sufficient hardness cannotbe achieved after quenching, but also uniform microstructure cannot beobtained. The heating temperature is therefore Ac3 transformation pointor more. If the heating temperature is more than 1300° C., an excessiveamount of energy is needed in the heating, which causes a decrease inproductivity. The heating temperature is therefore 1300° C. or less,preferably 1250° C. or less, more preferably 1200° C. or less, andfurther preferably 1150° C. or less.

Ac3 transformation point can be calculated using the following formula:

Ac3 (° C.)=912.0−230.5×C+31.6×Si−20.4×Mn−39.8×Cu−18.1×Ni−14.8×Cr+16.8×Mo

where each element symbol in the formula represents the content of thecorresponding element in mass%, with the content of each element notcontained being 0.

[Hot Rolling]

The heated steel material is then hot rolled to obtain a hot-rolledsteel plate. The hot rolling conditions are not limited, and the hotrolling may be performed by a conventional method. In the presentdisclosure, the hardness, etc. of the steel plate are controlled in theheat treatment process after the hot rolling, and accordingly the hotrolling conditions are not limited. However, from the viewpoint ofdecreasing the deformation resistance of the steel material and reducingthe load on the mill, the rolling finish temperature is preferably 750°C. or more, more preferably 800° C. or more, and further preferably 850°C. or more. From the viewpoint of preventing significant coarsening ofaustenite grains and the resulting decrease in ductility after the heattreatment, the rolling finish temperature is preferably 1000° C. or lessand more preferably 950° C. or less.

In the present disclosure, the hot-rolled steel plate is subjected toheat treatment including quenching. The heat treatment may be performedby any method of the below-described two embodiments. In the followingdescription, the term “cooling start temperature” refers to the surfacetemperature of the steel plate at the cooling start in the coolingprocess in quenching, and the term “cooling stop temperature” refers tothe surface temperature of the steel plate at the cooling end in thecooling process in quenching.

In one embodiment of the present disclosure, after the hot rolling, theobtained hot-rolled steel plate is subjected to quenching. The quenchingis performed by (a) direct quenching (DQ) or (b) reheating quenching(RQ). Although the method of cooling in the quenching is not limited,water cooling is preferable.

(a) Direct Quenching (DQ)

In the case of performing the quenching by direct quenching, thehot-rolled steel plate after the hot rolling is cooled from a coolingstart temperature that is Ar3 transformation point or more to a coolingstop temperature that is Mf point or less.

Cooling Start Temperature: Ar3 Transformation Point or More

If the cooling start temperature is Ar3 transformation point or more,the quenching starts from the austenite region, so that the desiredmartensite microstructure can be obtained. If the cooling starttemperature is less than Ar3 point, ferrite forms, causing the volumefraction of martensite in the finally obtained microstructure to be lessthan 90%. If the volume fraction of martensite is less than 90%, thehardness of the steel plate cannot be improved sufficiently, andconsequently the abrasion resistance of the steel plate decreases.Moreover, if the cooling start temperature is less than Ar3 point, adifference in hardness occurs in the transverse direction, so that thewide bending workability decreases. Although no upper limit is placed onthe cooling start temperature, the cooling start temperature ispreferably 950° C. or less.

Ar3 transformation point can be calculated using the following formula:

Ar3 (° C.)=910−273×C−74×Mn−57×Ni−16×Cr−9×Mo−5×Cu

where each element symbol in the formula represents the content of thecorresponding element in mass%, with the content of each element notcontained being 0.

Cooling Stop Temperature: Mf Point or Less

If the cooling stop temperature is more than Mf point, the volumefraction of martensite cannot be increased sufficiently, and the desiredhardness cannot be achieved. Moreover, if the cooling stop temperatureis more than Mf point, a difference in hardness occurs in the transversedirection, so that the wide bending workability decreases. The coolingstop temperature is therefore Mf point or less. The cooling stoptemperature is preferably (Mf point −100 ° C.) or less, more preferably(Mf point −120 ° C.) or less, and further preferably (Mf point −150 °C.) or less, from the viewpoint of increasing the volume fraction ofmartensite. Although no lower limit is placed on the cooling stoptemperature, the cooling stop temperature is preferably room temperatureor more because excessive cooling leads to lower production efficiency.

Mf point can be calculated using the following formula:

Mf(° C.)=410.5−407.3×C−7.3×Si−37.8×Mn−20.5×Cu−19.5×Ni−19.8×Cr−4.5×Mo

where each element symbol in the formula represents the content of thecorresponding element in mass%, with the content of each element notcontained being 0.

(b) Reheating Quenching (RQ)

In the case of performing the quenching by reheating quenching, first,the hot-rolled steel plate after the hot rolling is cooled, and thehot-rolled steel plate after the cooling is reheated to a reheatingtemperature that is Ac3 transformation point or more and 950° C. orless. The hot-rolled steel plate after the reheating is then cooled fromthe reheating temperature to a cooling stop temperature that is Mf pointor less.

Reheating Temperature: Ac3 Transformation Point or More and 950° C. orLess

Reheating the hot-rolled steel plate to Ac3 transformation point or morecan make the microstructure austenite, so that martensite microstructurecan be obtained by the subsequent quenching (cooling). If the reheatingtemperature is less than Ac3 transformation point, ferrite forms and thesteel plate is not sufficiently quenched, and consequently the hardnessof the steel plate cannot be sufficiently improved. This causes adecrease in the abrasion resistance of the finally obtained steel plate.The reheating temperature is therefore Ac3 transformation point or more.If the reheating start temperature is more than 950° C., crystal grainscoarsen and the workability decreases. The reheating temperature istherefore 950° C. or less. To start the cooling from the reheatingtemperature, for example, the cooling is started immediately after thehot-rolled steel plate is discharged from the furnace used for thereheating.

Cooling Stop Temperature: Mf Point or Less

If the cooling stop temperature is more than Mf point, the volumefraction of martensite cannot be increased sufficiently, and the desiredhardness cannot be achieved. Moreover, if the cooling stop temperatureis more than Mf point, a difference in hardness occurs in the transversedirection, so that the wide bending workability decreases. The coolingstop temperature is therefore Mf point or less. The cooling stoptemperature is preferably (Mf point −100° C.) or less, more preferably(Mf point −120° C.) or less, and further preferably (Mf point −150° C.)or less, from the viewpoint of increasing the volume fraction ofmartensite. Although no lower limit is placed on the cooling stoptemperature, the cooling stop temperature is preferably room temperatureor more because excessive cooling leads to lower production efficiency.

(Average Cooling Rate During Quenching)

The cooling rate in the cooling process in the quenching is not limited,and may be any cooling rate with which martensite phase forms. Forexample, the average cooling rate from the quenching start to thequenching stop is preferably 10° C./s or more, more preferably 15° C./sor more, and further preferably 20° C./s or more. Since a higher averagecooling rate during the quenching is better in principle, no upper limitis placed on the average cooling rate. However, given that a highercooling rate requires a cooling line capable of cooling at the coolingrate, the average cooling rate is preferably 150° C./s or less, morepreferably 100° C./s or less, and further preferably 80° C./s or less.The average cooling rate herein denotes the average cooling rate of thesurface temperature at the center position of the steel plate in thetransverse direction. The surface temperature can be measured using aradiation thermometer or the like.

(Cooling Rate Difference)

In the present disclosure, in the cooling process in the quenching, thedifference in average cooling rate between the center position and the ¼position of the hot-rolled steel plate in the transverse direction andthe difference in average cooling rate between the center position andthe ¾ position of the hot-rolled steel plate in the transverse directionare each 5° C./s or less. If the difference in average cooling rate(hereafter also referred to as “cooling rate difference”) is more than5° C./s, the difference in Vickers hardness between adjacent two pointsis more than 30Hv10, and the wide bending workability degrades. Theaverage cooling rate herein denotes the average cooling rate of thesurface temperature of the steel plate. The surface temperature can bemeasured using a radiation thermometer or the like.

(Tempering)

In one embodiment of the present disclosure, the quenched hot-rolledsteel plate may be optionally further subjected to tempering. Thetempering can further improve the uniformity of the hardness of thesteel plate. In the case of performing the tempering, the cooling stoptemperature in the quenching is preferably less than (Mf point −100°C.). After stopping the cooling at the cooling stop temperature, thesteel plate is heated to the below-described tempering temperature.

Tempering Temperature: (Mf Point −80° C.) or More and (Mf Point +50° C.)or Less

If the tempering temperature is less than (Mf point −80° C.), thetempering effect cannot be achieved. Accordingly, in the case ofperforming the tempering, the tempering temperature is (Mf point −80°C.) or more, preferably (Mf point −60° C.) or more, and more preferably(Mf point −50° C.) or more. If the tempering temperature is more than(Mf point +50° C.), the surface hardness decreases noticeably.Accordingly, in the case of performing the tempering, the temperingtemperature is (Mf point +50° C.) or less, preferably (Mf point +30° C.)or less, and more preferably (Mf point +10° C.) or less.

Temperature Holding

After the tempering temperature is reached, the heating can be stopped.In one embodiment of the present disclosure, however, after the heatingto the tempering temperature, the steel plate may be held at thetempering temperature for any holding time. The holding time is notlimited, but is preferably 60 sec or more and more preferably 5 min ormore from the viewpoint of enhancing the tempering effect. If theholding time is excessively long, the hardness of the steel plate maydecrease. Accordingly, in the case of performing the temperatureholding, the holding time is preferably 60 min or less, more preferably30 min or less, and further preferably 20 min or less.

Heating Rate

The heating rate to the tempering temperature in the tempering is notlimited. The average heating rate to the tempering temperature ispreferably 0.1° C./s or more and more preferably 0.5° C./s or more, fromthe viewpoint of productivity. If the average heating rate is 2° C./s ormore, carbide precipitates finely, with it being possible to furtherimprove the wide bending workability. Hence, the average heating rate ispreferably 2° C./s or more and more preferably 10° C./s or more, fromthe viewpoint of further improving the wide bending workability.Although no upper limit is placed on the average heating rate, anexcessively high heating rate requires a larger line for reheating andalso causes an increase in energy consumption. The average heating rateis therefore preferably 30° C./s or less, and more preferably 25° C./sor less.

The heating in the tempering is not limited, and may be performed by anymethod. For example, at least one method selected from the groupconsisting of heating using a heat treatment furnace, high frequencyinduction heating, and electrical resistance heating may be used. In thecase of performing the temperature holding, it is preferable to performthe reheating and the temperature holding using a heat treatmentfurnace. In the case where the average heating rate is 2° C./s or more,it is preferable to perform the heating to the tempering temperature byhigh frequency induction heating or electrical resistance heating. Inthe case of using the heat treatment furnace, the average heating rateis preferably 10° C./s or less. The tempering may be performed eitheroffline or online.

After heating to the tempering temperature and optionally holding thetemperature, the heating or the temperature holding is stopped. Thesubsequent cooling method is not limited, and may be one or both of aircooling and water cooling. In one embodiment of the present disclosure,after stopping the heating or the temperature holding, the steel platemay be allowed to naturally cool to room temperature.

In another embodiment of the present disclosure, the cooling in thequenching is stopped in a specific temperature range, and then aircooling is performed. The steel plate is thus tempered, so that theuniformity of the hardness of the steel plate can be further improved asin the case of performing the tempering in the foregoing embodiment.This embodiment will be described below.

Cooling Stop Temperature: Mf Point or Less and (Mf Point −100° C.) orMore

If the cooling stop temperature in the quenching is more than Mf point,the volume fraction of martensite cannot be increased sufficiently andthe desired hardness cannot be achieved, as mentioned above. Moreover,if the cooling stop temperature is more than Mf point, a difference inhardness occurs in the transverse direction, so that the wide bendingworkability decreases. The cooling stop temperature is therefore Mfpoint or less. If the cooling stop temperature is less than (Mf point−100° C.), the tempering effect cannot be achieved even when air coolingis performed after the cooling stop. Hence, in this embodiment, thecooling stop temperature is (Mf point −100° C.) or more. The coolingstop temperature is preferably (Mf point −80° C.) or more and morepreferably (Mf point −50° C.) or more, from the viewpoint of enhancingthe tempering effect by air cooling.

In this embodiment, the tempering effect can be achieved by performingair cooling after stopping the cooling at the cooling stop temperature.The air cooling is not limited and may be performed under anyconditions, but the cooling rate is preferably 1° C./s or less.

EXAMPLES

To demonstrate the effects of the presently disclosed techniques,abrasion-resistant steel plates were produced by the procedure describedbelow and their properties were evaluated.

First, molten steels having the chemical compositions listed in Table 1were produced through smelting, and steel slabs as steel materials wereobtained. Each obtained steel slab was heated to the heating temperatureshown in Table 2, and then hot rolled under the conditions shown inTable 2 to obtain a hot-rolled steel plate. The obtained hot-rolledsteel plate was subjected to direct quenching or reheating quenchingunder the conditions shown in Table 2, to produce an abrasion-resistantsteel plate. In some examples, after the quenching, tempering wasperformed under the conditions shown in Table 2. In each example withouttempering, after the quenching stop, air cooling was performed at acooling rate of 1° C./s or less.

The column “cooling rate difference” in Table 2 shows the larger valueout of the difference in average cooling rate between the centerposition and the ¼ position of the hot-rolled steel plate in thetransverse direction and the difference in average cooling rate betweenthe center position and the ¾ position of the hot-rolled steel plate inthe transverse direction in the cooling process in the quenching.

For each obtained abrasion-resistant steel plate, the volume fraction ofmartensite (M), the hardness, the maximum transverse direction hardnessdifference, and the wide bending radius were evaluated. The evaluationmethods are as follows.

(Volume Fraction of Martensite)

A sample was collected from each steel plate so that a position of 1 mmin depth from the surface of the steel plate would be the observationposition. The surface of the sample was mirror polished and furthernital etched, and then a range of 10 mm×10 mm was photographed using ascanning electron microscope (SEM). The captured image was analyzedusing an image analyzer to determine the area fraction of martensite.Ten observation fields were observed at random, and the average value ofthe obtained area fractions was taken to be the volume fraction ofmartensite.

(Surface Hardness)

A hardness measurement test piece was collected from each obtainedabrasion-resistant steel plate, and the Brinell hardness was measured inaccordance with JIS Z 2243 (1998). To exclude the influence of scale anda decarburized layer present at the surface of the abrasion-resistantsteel plate, the measurement was performed after removing a region fromthe steel plate surface to a depth of 1 mm by grinding. Hence, themeasured hardness was the hardness in a plane of 1 mm in depth from thesteel plate surface. The measurement position in the transversedirection was a position of ¼ of the plate width (i.e. ¼ position in thetransverse direction). In the measurement, a tungsten hard ball of 10 mmin diameter was used, and the load was 3000 kgf.

(Transverse Direction Hardness Difference)

The Vickers hardness at a depth of 1 mm from the surface of eachabrasion-resistant steel plate was measured at intervals of 10 mm in thetransverse direction. In the measurement, a region of 50 mm on each endof the transverse direction of the abrasion-resistant steel plate wasexcluded from the measurement range. From the obtained values, theabsolute difference in Vickers hardness between adjacent two points wascalculated. The maximum value of the absolute differences is shown inTable 3. The test load in the measurement of the Vickers hardness was 10kg.

(Limit Bending Radius)

A bending test piece of 200 mm in width and 300 mm in length wascollected from each obtained steel plate, and a bending test with abending angle of 180° was conducted in accordance with JIS Z 2248. Fromthe minimum bending radius R (mm) without cracking and the platethickness t (mm) in the bending test, the limit bending radius R/t wascalculated.

The evaluation results obtained by these methods are listed in Table 3.As can be understood from the results in Table 3, eachabrasion-resistant steel plate satisfying the conditions according tothe present disclosure had a surface hardness of 420 HBW 10/3000 or morein Brinell hardness and was excellent in abrasion resistance. Eachabrasion-resistant steel plate satisfying the conditions according tothe present disclosure also had a limit bending radius R/t of 5.0 orless in the bending test, exhibiting favorable wide bending workability.Thus, each abrasion-resistant steel plate according to the presentdisclosure was excellent in both abrasion resistance and wide bendingworkability. These results demonstrate that the presently disclosedtechniques can improve the wide bending workability without a decreasein the surface hardness of the abrasion-resistant steel plate.

TABLE 1 Steel sample Chemical composition (mass %) * ID C Si Mn P S AlCr N Nb Ti B Cu Ni Mo A 0.160 0.75 1.35 0.008 0.0012 0.043 0.26 0.00290.007 0.012 0.0009 0.20 0.15 0.20 B 0.173 0.48 1.53 0.006 0.0022 0.0360.45 0.0052 0.011 0.008 — 0.12 — — C 0.210 0.86 0.75 0.004 0.0009 0.0260.33 0.0041 0.013 0.006 0.0015 0.13 0.55 0.56 D 0.233 0.16 0.63 0.0070.0006 0.028 0.19 0.0070 — — 0.0013 — 2.10 — E 0.245 0.24 1.03 0.0050.0010 0.026 0.55 0.0028 — — — — — — F 0.262 0.26 0.73 0.004 0.00070.030 0.38 0.0033 0.018 0.012 0.0009 — — 0.16 G 0.270 0.22 0.54 0.0140.0068 0.055 0.83 0.0045 0.014 0.015 0.0025 — — — H 0.272 0.38 0.870.005 0.0005 0.024 0.74 0.0028 0.012 0.014 0.0012 — — 0.38 I 0.293 0.070.71 0.003 0.0008 0.029 0.78 0.0035 — — — — — — J 0.120 0.32 0.85 0.0040.0015 0.032 0.63 0.0041 — 0.013 — — — 0.50 K 0.330 0.45 0.98 0.0120.0026 0.041 0.18 0.0036 — — — — — — L 0.255 0.02 1.01 0.008 0.00100.036 0.26 0.0041 0.012 — 0.0009 — — — M 0.231 1.25 0.86 0.005 0.00650.028 0.15 0.0036 — 0.013 0.0015 0.05 0.12 — N 0.282 0.36 2.50 0.0080.0006 0.036 0.13 0.0029 — — — — 1.30 0.30 O 0.155 0.86 0.35 0.0050.0026 0.045 0.33 0.0063 0.008 0.009 0.0013 0.45 0.43 — P 0.258 0.450.95 0.045 0.0038 0.032 0.24 0.0052 — — — — — — Q 0.192 0.23 0.89 0.0160.0185 0.016 0.56 0.0087 0.009 0.019 0.0022 — — — R 0.188 0.36 1.150.012 0.0015 0.088 0.63 0.0025 — — — — — — S 0.195 0.25 1.12 0.0090.0009 0.004 0.25 0.0033 0.015 — — 0.36 0.38 — T 0.275 0.31 1.03 0.0060.0007 0.026 1.18 0.0045 0.013 — — — — — U 0.263 0.45 1.37 0.008 0.00150.035 0.03 0.0029 — — — — — — V 0.241 0.26 1.59 0.012 0.0028 0.019 0.180.0128 — 0.018 0.0015 — — — W 0.190 0.35 0.70 0.008 0.0009 0.025 0.730.0040 0.018 0.013 0.0009 0.02 0.03 0.13 X 0.251 0.18 1.89 0.005 0.00080.018 0.13 0.0024 — — — — — — Steel sample Chemical composition (mass%) * Ar3 Ac3 Mf Ms ID V W Co Ca Mg REM (° C.) (° C.) (° C.) (° C.)Remarks A — — — 0.0012 — — 751 860 276 417 Conforming steel B — — — —0.0025 — 742 845 267 410 Conforming steel C — — 0.26 — — 0.0108 755 865268 403 Conforming steel D — 0.33 — — — — 677 810 246 381 Conformingsteel E — — — — — — 758 834 259 396 Conforming steel F — — — — — — 777842 266 398 Conforming steel G 0.042 — — 0.0024 — — 783 833 262 397Conforming steel H 0.024 — — — — — 756 839 248 383 Conforming steel I —— — — — — 765 821 248 384 Conforming steel J — — — — — — 800 876 312 449Comparative steel K 0.015 — — 0.0023 — — 745 827 232 364 Comparativesteel L — — — — — — 761 829 263 397 Comparative steel M — — — — — — 774874 268 402 Comparative steel N — — — — — — 569 787 169 314 Comparativesteel O — — — 0.0025 — — 810 866 304 445 Comparative steel P — — — — — —765 844 261 395 Comparative steel Q — — — — — — 783 849 286 422Comparative steel R — — — — — — 763 847 275 414 Comparative steel S0.025 — — — — — 746 827 267 411 Comparative steel T — — — — — — 740 820234 375 Comparative steel U — — — — — — 736 837 248 382 Comparativesteel V — — — 0.0018 — — 724 830 247 385 Comparative steel W — — — — — —792 855 288 424 Conforming steel X — — — — — — 700 819 233 372Conforming steel * Balance consists of Fe and inevitable impurities.

TABLE 2 Direct quenching Reheating Hot rolling Cooling quenching SteelHeating Rolling Cooling Cooling Cooling Average Reheating Steel materialHeating finish Plate start stop rate cooling Reheating sample Thicknesstemperature temperature thickness temperature temperature differencerate temperature No. ID (mm) (° C.) (° C.) (mm) (° C.) (° C.) (° C./s)(° C./s) (° C.) 1 A 250 1200 850 25 — — — — 900 2 B 250 1150 850 25 — —— — 900 3 W 250 1150 850 50 — — — — 900 4 C 250 1150 850 25 820 250 2 30— 5 D 250 1250 750 6 730 100 2 50 — 6 E 250 1100 850 25 820 200 3 40 — 7E 250 1100 850 25 820 100 3 40 — 8 X 250 1100 850 25 — — — — 900 9 F 2501150 900 12 — — — — 900 10 F 250 1100 900 25 850 240 2 30 — 11 F 2501150 850 25 — — — — 900 12 G 250 1150 900 25 — — — — 950 13 G 250 1150900 25 — — — — 950 14 H 250 1100 850 60 — — — — 900 15 I 300 1150 900 50— — — — 900 16 J 300 1200 900 50 850 200 3 20 — 17 K 300 1150 850 40 — —— — 900 18 L 200 1150 900 30 — — — — 900 19 M 200 1150 850 25 — — — —950 20 N 250 1050 750 60 730 150 3 15 — 21 O 250 1100 900 25 850 200 530 — 22 P 250 1150 900 25 — — — — 900 23 Q 250 1050 900 25 850 250 3 40— 24 R 250 1100 900 25 — — — — 900 25 S 250 1100 850 25 — — — — 900 26 T300 1100 850 25 — — — — 900 27 U 300 1100 850 25 — — — — 900 28 V 3001100 800 25 — — — — 900 29 I 250 1100 800 25 750 200 2 30 — 30 I 2501100 850 25 800 300 2 30 — 31 I 250 1150 900 25 — — — — 800 32 I 2501100 850 25 — — — — 900 33 I 250 1100 850 25 800 200 7 10 — 34 I 2501150 900 25 — — — — 900 Reheating quenching Cooling Cooling CoolingAverage Tempering stop rate cooling Tempering Heating Holdingtemperature difference rate temperature Heating rate time No. (° C.) (°C./s) (° C./s) (° C.) method (° C./s) (min.) Remarks 1 240 1 40 — — — —Example 2 240 2 40 — — — — Example 3 200 1 20 — — — — Example 4 — — — —— — — Example 5 — — — 200 Heating 3 20 Example furnace 6 — — — — — — —Example 7 — — — — — — — Example 8 100 — — — — — — Example 9 200 0 60 — —— — Example 10 — — — — — — — Example 11 220 1 40 — — — — Example 12 1001 40 200 Induction 7  1 Example 13 100 1 40 — — — — Example 14 100 3 15— — — — Example 15 100 2 20 180 Heating 5 10 Example furnace 16 — — — —— — — Comparative Example 17 100 3 20 — — — — Comparative Example 18 2002 30 — — — — Comparative Example 19 100 1 40 — — — — Comparative Example20 — — — — — — — Comparative Example 21 — — — — — — — ComparativeExample 22 100 — — — — — — Comparative Example 23 — — — — — — —Comparative Example 24 200 2 40 — — — — Comparative Example 25 200 2 30— — — — Comparative Example 26 200 2 30 — — — — Comparative Example 27240 2 30 — — — — Comparative Example 28 100 2 30 — — — — ComparativeExample 29 — — — — — — — Comparative Example 30 — — — — — — —Comparative Example 31 200 2 40 — — — — Comparative Example 32 300 — — —— — — Comparative Example 33 — — — — — — — Comparative Example 34 200 810 — — — — Comparative Example

TABLE 3 Measurement results Hardness Volume fraction Hardness (HBW of Mdifference No. 10/3000) (%) (HV10) R/t Remarks 1 455 90 18 3.5 Example 2468 90 16 4.0 Example 3 428 90 10 4.0 Example 4 485 90 21 4.0 Example 5503 95 15 4.5 Example 6 512 95 14 4.5 Example 7 528 95 18 5.0 Example 8513 95 12 5.0 Example 9 536 95 22 5.0 Example 10 503 90 15 4.5 Example11 513 90 13 4.5 Example 12 529 95 20 4.5 Example 13 540 95 22 5.0Example 14 541 95 19 5.0 Example 15 548 95 18 4.5 Example 16 415 70 113.0 Comparative Example 17 583 95 15 6.5 Comparative Example 18 516 9022 7.0 Comparative Example 19 538 90 14 8.0 Comparative Example 20 58295 26 6.0 Comparative Example 21 418 75 45 4.0 Comparative Example 22524 90 13 6.5 Comparative Example 23 488 90 23 6.5 Comparative Example24 469 95 26 7.0 Comparative Example 25 475 90 21 6.5 ComparativeExample 26 521 90 23 6.5 Comparative Example 27 481 75 53 6.0Comparative Example 28 509 90 20 7.0 Comparative Example 29 405 70 846.0 Comparative Example 30 418 70 92 7.0 Comparative Example 31 378 6559 6.0 Comparative Example 32 393 65 88 7.5 Comparative Example 33 50375 85 7.0 Comparative Example 34 498 85 73 7.5 Comparative Example

1. An abrasion-resistant steel plate comprising a chemical compositioncontaining, in mass%, C: 0.15% to 0.30%, Si: 0.05% to 1.00%, Mn: 0.50%to 2.00%, P: 0.020% or less, S: 0.010% or less, Al: 0.01% to 0.06%, Cr:0.10% to 1.00%, and N: 0.0100% or less, with a balance consisting of Feand inevitable impurities, wherein a volume fraction of martensite at adepth of 1 mm from a surface of the abrasion-resistant steel plate is90% or more, hardness at a depth of 1 mm from the surface is 420 HBW10/3000 to 560 HBW 10/3000 in Brinell hardness, and a transversedirection hardness difference is 30Hv10 or less in Vickers hardness, thetransverse direction hardness difference being defined as a differencein the hardness at a depth of 1 mm from the surface between two pointsadjacent at intervals of 10 mm in a transverse direction of theabrasion-resistant steel plate.
 2. The abrasion-resistant steel plateaccording to claim 1, wherein the chemical composition further contains,in mass%, at least one of the groups consisting of a) one or moreselected from the group consisting of Nb: 0.005% to 0.020%, Ti: 0.005%to 0.020%, and B: 0.0003% to 0.0030%, b) one or more selected from thegroup consisting of Cu: 0.01% to 0.5%, Ni: 0.01% to 3.0%, Mo: 0.1% to1.0%, V: 0.01% to 0.10%, W: 0.01% to 0.5%, and Co: 0.01% to 0.5%, and c)one or more selected from the group consisting of Ca: 0.0005% to0.0050%, Mg: 0.0005% to 0.0100%, and REM: 0.0005% to 0.0200%. 3-4.(canceled)
 5. A method of producing an abrasion-resistant steel plate,the method comprising: heating a steel material to a heating temperaturethat is an Ac3 transformation point or more and 1300° C. or less, thesteel material having a chemical composition containing, in mass%, C:0.15% to 0.30%, Si: 0.05% to 1.00%, Mn: 0.50% to 2.00%, P: 0.020% orless, S: 0.010% or less, Al: 0.01% to 0.06%, Cr: 0.10% to 1.00%, and N:0.0100% or less, with a balance consisting of Fe and inevitableimpurities; hot rolling the heated steel material to obtain a hot-rolledsteel plate; and subjecting the hot-rolled steel plate to quenching,wherein the quenching is (a) direct quenching of cooling the hot-rolledsteel plate from a cooling start temperature that is an Ar3transformation point or more to a cooling stop temperature that is a Mfpoint or less or (b) reheating quenching of cooling the hot-rolled steelplate, reheating the cooled hot-rolled steel plate to a reheatingtemperature that is the Ac3 transformation point or more and 950° C. orless, and cooling the reheated hot-rolled steel plate from the reheatingtemperature to a cooling stop temperature that is the Mf point or less,and in a cooling process in the quenching, a difference in averagecooling rate between a center position and a ¼ position of thehot-rolled steel plate in the transverse direction and a difference inaverage cooling rate between the center position and a ¾ position of thehot-rolled steel plate in the transverse direction are each 5° C./s orless.
 6. The method of producing an abrasion-resistant steel plateaccording to claim 5, wherein the cooling stop temperature in thequenching is less than (Mf point −100° C.), and the method comprises,after the quenching, tempering the quenched hot-rolled steel plate at atempering temperature that is (Mf point −80° C.) or more and (Mf point+50° C.) or less.
 7. The method of producing an abrasion-resistant steelplate according to claim 6, wherein in the tempering, the quenchedhot-rolled steel plate is held at the tempering temperature for 60 s ormore.
 8. The method of producing an abrasion-resistant steel plateaccording to claim 6 or 7, wherein an average heating rate in thetempering is 2° C./s or more.
 9. The method of producing anabrasion-resistant steel plate according to claim 5, wherein the coolingstop temperature in the quenching is Mf point or less and (Mf point−100° C.) or more, and the method comprises, after the quenching, aircooling the quenched hot-rolled steel plate.
 10. The method of producingan abrasion-resistant steel plate according to claim 5, wherein thechemical composition further contains, in mass%, at least one of thegroups consisting of a) one or more selected from the group consistingof Nb: 0.005% to 0.020%, Ti: 0.005% to 0.020%, and B: 0.0003% to0.0030%, b) one or more selected from the group consisting of Cu: 0.01%to 0.5%, Ni: 0.01% to 3.0%, Mo: 0.1% to 1.0%, V: 0.01% to 0.10%, W:0.01% to 0.5%, and Co: 0.01% to 0.5%, and c) one or more selected fromthe group consisting of Ca: 0.0005% to 0.0050%, Mg: 0.0005% to 0.0100%,and REM: 0.0005% to 0.0200%. 11-12. (canceled)
 13. The method ofproducing an abrasion-resistant steel plate according to claim 6,wherein the chemical composition further contains, in mass%, at leastone of the groups consisting of a) one or more selected from the groupconsisting of Nb: 0.005% to 0.020%, Ti: 0.005% to 0.020%, and B: 0.0003%to 0.0030%, b) one or more selected from the group consisting of Cu:0.01% to 0.5%, Ni: 0.01% to 3.0%, Mo: 0.1% to 1.0%, V: 0.01% to 0.10%,W: 0.01% to 0.5%, and Co: 0.01% to 0.5%, and c) one or more selectedfrom the group consisting of Ca: 0.0005% to 0.0050%, Mg: 0.0005% to0.0100%, and REM: 0.0005% to 0.0200%.
 14. The method of producing anabrasion-resistant steel plate according to claim 9, wherein thechemical composition further contains, in mass%, at least one of thegroups consisting of a) one or more selected from the group consistingof Nb: 0.005% to 0.020%, Ti: 0.005% to 0.020%, and B: 0.0003% to0.0030%, b) one or more selected from the group consisting of Cu: 0.01%to 0.5%, Ni: 0.01% to 3.0%, Mo: 0.1% to 1.0%, V: 0.01% to 0.10%, W:0.01% to 0.5%, and Co: 0.01% to 0.5%, and c) one or more selected fromthe group consisting of Ca: 0.0005% to 0.0050%, Mg: 0.0005% to 0.0100%,and REM: 0.0005% to 0.0200%.